1. Field Of The Invention
This invention relates to differential amplifiers and, more particularly, to circuitry utilizing DC operational amplifiers to form a composite differential direct-coupled instrumentation amplifier having both high DC accuracy and wide bandwidth.
2. History Of The Prior Art
A typical prior art instrumentation amplifier is implemented with three operational amplifiers. Two of the operational amplifiers each receive one of the two differential input signals the difference between which is to be measured; these two operational amplifiers each provide high closed loop gains. The two output signals generated by these operational amplifiers are furnished as the input signals to the third operational amplifier which utilizes balanced feedback and input resistors so that it operates at a closed loop gain of one. Because it operates at a gain of one, it may provide a relatively high bandwidth of approximately one megahertz and little DC error.
Several types of operational amplifiers may be used in such a circuit with varying performance characteristics. However, devices which provide good DC performance do not provide closed loop bandwidths above a few kilohertz. Because of their limited bandwidth, they also exhibit poor AC characteristics and have poor AC common mode rejection at frequencies as low as 60 hertz. Conversely, operational amplifiers which can provide closed-loop bandwidths approaching one megahertz have DC errors several orders of magnitude higher than the precision DC devices.
A high performance instrumentation amplifier, intended for multi-frequency use, should provide both good DC performance as well as frequency response and well controlled phase shift characteristics from DC to beyond 500 kilohertz. By comparing these statistics with those of a typical prior art instrumentation amplifier, it may be seen that the typical instrumentation amplifier does not provide these results.